1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a safety vent which has an asymmetric structure able to accommodate minor deformation and that is not susceptible to local cracking, but which will quickly separate from the structure encasing the battery and provide a substantial opening for the battery to rapid vent to atmosphere in order to prevent the secondary battery from bursting when the internal pressure of a battery increases excessively or external impact is applied thereto.
2. Description of Related Art
In general, secondary batteries are rechargeable batteries having a small size and high capacity. In recent years, the secondary batteries, for example, nickel-metal hydride (Ni-MH) batteries and lithium batteries have come into widespread use.
The secondary batteries are the main parts used for various portable electronic apparatuses, such as cellular phones, notebook computers, and PDAs. The market of small secondary batteries has been gradually expanded with the improvements of performances of portable electronic apparatuses and proliferation of portable electronic apparatuses.
Particularly, in lithium secondary batteries, a lithium-based oxide is used as a positive electrode active material, and a carbon material is used as a negative electrode active material. The lithium secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries according to the type of electrolyte. That is, batteries using liquid electrolytes are called lithium ion (Li-ion) batteries, and batteries using polymer electrolytes are called lithium polymer batteries. In addition, the lithium secondary batteries are manufactured in various shapes, such as a cylindrical shape, a square shape, and a pouch shape.
Among the lithium secondary batteries, in the case of the lithium ion battery, when the lithium ion battery is overcharged, the resolution of the electrolyte occurs in the positive electrode, and lithium metal is extracted from the negative electrode, and these phenomena may cause the characteristics of the battery to deteriorate or the battery to be fired or to generate heat. In the case of the lithium polymer battery, when the lithium polymer battery is locally overheated during charging or discharging, the polymer electrolyte having low heat resistance is partially resolved or softened, and this phenomenon causes a current or potential to be non-uniform and results in a short circuit. As a result, the battery may be burst or exploded. In addition, the internal pressure of the battery increases when gas is generated during an electrical/chemical reaction in the battery, thus the battery is expanded and the expansion may cause the explosion of the battery. These phenomena are fatal to the security of the battery.
Therefore, in order to solve these problems, various techniques for ensuring the safety of the battery have been proposed. For example, a battery having a safety vent for discharging gas generated in the battery to the exterior at a pressure higher than a predetermined value has been proposed. In general, the safety vent is manufactured by forming a groove in a portion of a cap plate of a secondary battery or in a portion of a can with a constant thickness by using a mechanical method, an etching method, or an electroforming method. The groove formed in a portion of the cap plate or the can is generally a notch.
A hot plate test is performed as one of the safety tests for a secondary battery having the safety vent. In the hot plate test, a manufactured secondary battery (generally, a bare cell) is placed on the plate heated at a temperature of 200° C. to 250° C. to test the resistance of the battery against high temperature.
Further, a drop test is performed as another test to the safety of the secondary battery, and the drop test is an endurance test for external impact. In the drop test, for example, a secondary battery is put into a pipe having a height of 1 m and the pipe is rotated for a predetermined amount of time.
In the safety vent according to the related art, the notch, which is a burst portion, is symmetric with respect to a virtual center-line thereof. Different pressures may be applied to portions of the notch. For example, when the notch is formed in the cap plate, different pressures may be applied to a portion of the notch close to an electrode terminal provided at the center of the cap plate and another portion of the notch away from the electrode terminal. Therefore, when the internal pressure of the secondary battery increases due to a short circuit in the secondary battery or the overcharge thereof, in many cases, a local crack, occurs at the notch of the safety vent. The local crack occurring in the safety vent causes an abnormal state of the battery. As a result, the safety vent is not completely exploded at an internal pressure higher than a predetermined value and thus does not reduce the increased internal pressure of the battery. As such, when the safety vent abnormally operates, i.e., does not completely explodes, the internal pressure of the battery continuously increases and results in the explosion of the battery.
Further, when external impact is applied to the battery due to the drop of the battery, a greater impact may be applied to both sides of the cross section of the battery than to the center of the cross section of the battery. As described above, in the related art, since the safety vent is symmetric with respect to the virtual center line thereof, a greater impact is applied to portions of the safety vent close to both sides of the battery, which may cause defects of a complete burst, such as a local crack.
Furthermore, when heat is applied to the battery from outside, for example, in the structure in which a safety vent is formed in the cap plate of the battery, the cap plate is heated from a circumferential portion to a central portion. In the safety vent according to the related art that is symmetric with respect to the virtual center line, a portion of the safety vent adjacent to the edge of the cap plate takes up a considerable area of the overall area of the safety vent, and defects of a complete burst, such as a local crack, is more likely to occur in a portion of the safety vent close to the edge of the cap plate.